Application of Supercapacitors in Distributed Microgrid

With the development of renewable energy power generation technology, microgrids capable of integrating distributed power sources are an effective way to meet increasing power demand, save investment and improve energy efficiency. As an energy buffering part of the microgrid, the energy storage system plays an increasingly important role. The characteristics and performance of supercapacitors are summarized. The structure and control principle of supercapacitor energy storage system are analyzed, and its application in microgrid is elaborated. The supercapacitor-based energy storage system not only functions as an energy buffer, but also provides short-time power supply, buffers load fluctuations in the microgrid, equalizes micro-power output, improves microgrid power quality, and is important for the economic performance of the microgrid. effect.

Today's society has higher and higher requirements for the quality and safety and reliability of energy and power supply. The traditional large-grid power supply method cannot meet this requirement because of its own defects. A new type of power grid that can integrate distributed power generation – microgrid emerges as the times require. It can save investment, reduce energy consumption, improve system security and flexibility, and is the future development direction. As an indispensable energy storage system in the microgrid, it plays a very important role. As a new type of energy storage device, supercapacitor is one of the preferred devices for microgrid energy storage because of its irreplaceable superiority.

At present, in the relatively remote mountainous areas of China, the cost of erecting transmission lines is relatively high, and even if the transmission lines are erected, the operating costs are high, so it is difficult to achieve electrification. If wind power or solar power is used to build a microgrid, the electric field that converts electricity into a supercapacitor can be stored. It is very economical to convert the electric field energy into electrical energy when needed, without any damage to the environment.

For most rural areas in China, the reliability of the power grid is often not high, and short-term power outages are inevitable. However, the cost of improving reliability is too high. The microgrid can be built up in the load concentration area, and the power is stored by the supercapacitor energy storage system when the power is normally supplied, and is supplied by the supercapacitor energy storage system during the power outage. Even in China's more developed urban areas, supercapacitor energy storage systems play an important role. The supercapacitor energy storage system stores the power when the power is sufficient, and feeds back to the grid when the power supply is insufficient, ensuring that the grid load is always balanced. At the same time, the supercapacitor energy storage system can improve the power quality, replace the currently used UPS, and improve the reliability of power supply for important load devices such as communication devices, computers and medical equipment.

It can be seen that the ultra-capacitor energy storage system which is economical and environmentally friendly has great market prospects. The application of the supercapacitor energy storage system in the microgrid also meets the requirements for environmental protection. Non-polluting energy sources such as solar energy, wind energy and fuel cells are stored in supercapacitors, providing electricity in a timely manner. There is no need to invest in large power stations or complex transmission networks. It is a kind of investment that can effectively use renewable energy. Energy saving measures.

1. Supercapacitor energy storage

1.1 Characteristics of supercapacitors

Supercapacitor, also known as Electrical Double-Layer Capacitor, Gold Capacitor, Farad Capacitor, is a new type of energy storage component between traditional capacitors and rechargeable batteries, with a capacity of several hundred to tens of thousands. The method has a specific power of more than 10 times that of the battery, and has a higher storage capacity than ordinary capacitors. It has a wide operating temperature range, rapid charge and discharge, long cycle life, no pollution, and zero emissions.

1) Has an extremely high capacity. The capacity of supercapacitors can be up to 10,000 FF, which is 2000-6000 times larger than the capacity of electrolytic capacitors of the same volume.

2) Has a very high power density. Supercapacitors have a power density of around 18 kW/kg and can discharge hundreds to thousands of amps in a short period of time. This feature makes supercapacitors ideal for short-term, high-power output applications.

3) The charging and discharging speed is fast. Supercapacitors do not produce a chemical reaction and can store electricity directly. The time required for charging is very short. Supercapacitor charging can be charged with a large current, and the charging process can be completed in several tens of seconds to several minutes, which is a true fast charging. The battery takes a few hours to complete the charging, and it takes dozens of minutes to use the fast charging.

4) Long service life. Supercapacitors are semi-permanent and do not need to be replaced. After repeated charging and discharging of the conventional rechargeable battery, the electrolyte gradually decomposes, the material deteriorates, and the performance also decreases. After a few years, most of them need to be replaced. The electrochemical reactions occurring during charging and discharging of supercapacitors have good reversibility, and can be repeatedly charged and discharged hundreds of thousands of times or more, basically without replacement.

5) Ambient temperature has little effect on normal use. Wide temperature range and excellent low temperature performance, up to -40 to +85 °C. The battery is only 0 ~ +40 ° C.

Of course, supercapacitors also have their drawbacks. If the supercapacitor is used improperly, it may cause electrolyte leakage. Supercapacitors should not be used in high-frequency charging and discharging circuits. High-frequency rapid charging and discharging can cause internal heating of capacitors, capacity attenuation, increase of internal resistance, and in some cases, capacitor performance collapse. When supercapacitors are used in series, there is a voltage equalization problem between the cells. A simple series connection can cause overvoltage of one or several of the individual capacitors, which can damage these capacitors and affect overall performance.

1.2 Comparison with other energy storage components

Supercapacitor and battery, superconducting energy storage and flywheel energy storage performance comparison. The battery technology is mature and the price is low, but its low cycle life and pollution environment will soon be replaced by new environmentally friendly energy storage components. Flywheel energy storage, superconducting energy storage and supercapacitors are excellent energy storage components, which are the future development directions. They have similar characteristics and can be applied to the microgrid. Superconducting energy storage and flywheel energy storage can be used for fast compensation, but its power density is much lower than that of supercapacitors, and the effect is worse. Compared with other energy storage methods, superconducting energy storage is expensive, and in addition to the cost of superconducting itself, the cost of maintaining low temperature is considerable. The flywheel energy storage is limited by the speed and mechanical strength.

In the microgrid, power quality problems caused by loads or micro-power sources often have short duration and frequent occurrence. In comparison, as a short-term energy storage device, supercapacitors are more ideal. Therefore, the application of supercapacitors in the microgrid is mainly considered. Although the price of supercapacitors is still high, with the gradual decline of prices, supercapacitors as an efficient, practical and environmentally friendly energy storage device will inevitably become an ideal choice.

2. Supercapacitor operation and control principle

2.1 Structure of supercapacitor energy storage system

The basic structure of a supercapacitor energy storage system. The supercapacitor is mostly an electric double layer structure, and the space between the activated carbon electrode and the electrolyte is a spatially distributed structure, and the characteristics of the supercapacitor can be described by series and parallel connection of a plurality of capacitors.

During the charging and discharging process of the ultracapacitor group, the terminal voltage range changes greatly, and it is usually necessary to use a DC/DC converter as an interface circuit to regulate the energy storage and release energy of the supercapacitor. The DC/AC converter can be a bidirectional DC/AC inverter or an AC/DC rectifier and a DC/AC inverter. The supercapacitor energy storage system is connected in parallel to the bus or feeder in the microgrid.

The supercapacitor energy storage system uses multiple sets of supercapacitors to store energy in the form of electric field energy. When the energy is urgently lacking or needed, the stored energy is released through the control unit to accurately and quickly compensate the active and reactive power required by the system. In order to achieve power balance and stability control. The advantages of the supercapacitor itself make it a winner in competing with other energy storage methods when applied to distributed generation.

2.2 Supercapacitor operation in the microgrid

A typical structural diagram of a microgrid. As can be seen from Figure 2, the microgrid consists of micro power supply, load, energy storage and energy manager. The form of energy storage in the microgrid is: connected to the DC bus of the micro power supply, the feeder containing the important load or the AC bus of the microgrid. Among them, the first two can be called distributed energy storage, and the last one is called central energy storage.

When connected to the grid, the power fluctuations in the microgrid are balanced by the large grid, and the energy storage is in the charging standby state. When the microgrid is switched from grid-connected operation to isolated network operation, the central energy storage starts immediately to compensate for the power shortage. The fluctuation of the load during the running of the microgrid or the fluctuation of the micro power supply can be balanced by the central energy storage or the distributed energy storage. Among them, the power fluctuation of the micro power supply has two kinds of balance modes, that is, the distributed energy storage and the micro power supply that needs energy storage are connected to a certain feeder, or the energy storage is directly connected to the DC bus of the micro power supply.

2.3 Supercapacitor control principle

The control of the supercapacitor is mainly embodied in the control of the DC/DC converter and the DC/AC converter. In recent years, converter control technology has developed rapidly, from the earliest open-loop control to the output voltage instantaneous feedback control, from analog control to full digital control. Current digital control methods include digital PID control, state feedback control, fuzzy control, and neural network control. Among them, the digital PID control method is the most widely used control method in engineering practice.

The bidirectional DC/DC converter realizes energy conversion between the DC low side super capacitor bank and the DC high voltage side. The control target of the DC/DC converter cannot be simply set to maintain the constant voltage of the DC high-voltage side of the storage capacitor, and must also meet the power limitation of the supercapacitor.

3. Application of supercapacitors in microgrid

3.1 Provide short-term power supply

There are two typical modes of operation in the microgrid: under normal circumstances, the microgrid and the conventional distribution network are connected to the grid, which is called the grid-connected operation mode; when the grid fault is detected or the power quality is not met, the microgrid will be timely The grid is disconnected to operate independently,

It is called the isolated network operation mode. Microgrid often needs to absorb part of the active power from the conventional distribution network. Therefore, when the microgrid is switched from the grid-connected mode to the isolated mode, there will be power shortage. Installing the energy storage device will help the smooth transition of the two modes.

3.2 Used as an energy buffer

Due to the small scale of the microgrid, the inertia of the system is not large, and the network and load often fluctuate very much, which has an impact on the stable operation of the entire microgrid. We always expect high-efficiency generators (such as fuel cells) in the microgrid to always operate at its rated capacity. However, the load on the microgrid does not remain constant throughout the day. Instead, it fluctuates as the weather changes. In order to meet the peak load supply, peak load adjustment must be carried out using a fuel-and-gas peaking power plant. Due to the high fuel price, the operation cost of this method is too expensive.

The supercapacitor energy storage system can effectively solve this problem. It can store excess power of the power supply when the load is low, and feed back to the micro grid to adjust the power demand when the load is high. As an energy buffering part of the microgrid, the energy storage system plays an increasingly important role. It not only avoids the generator set installed to meet the peak load, but also makes full use of the power generation of the unit when the load is low, avoiding waste.

The high power density and high energy density of the supercapacitor make it the best choice for handling peak loads, and the use of supercapacitors requires only the storage of energy equivalent to the peak load. If battery storage is used, it is necessary to store several times the energy of the peak load. Batteries have been widely used as energy storage units, but frequent charging and discharging control is required in the microgrid, which will inevitably shorten the service life of the battery.

In a microgrid containing a malignant load such as an elevator, a hoist or a subway power station, the configuration of the supercapacitor energy storage unit can reduce the negative impact of the electric drive system on the microgrid. In a strong load system such as a motor or a transmission on the load side, when a large load suddenly starts, a large instantaneous current is generally required. At this time, if the power of the power source is insufficient, the power supply voltage will drop instantaneously, thereby causing the control circuit to generate Misoperation, if the power supply capacity is increased, it is obviously a waste for a work that does not require a large current. Adding a high-power supercapacitor to the system can drive a larger load with a smaller capacity.

3.3 Improve the power quality of the microgrid

People are paying more and more attention to power quality issues. On the one hand, the microgrid as the power grid must meet the requirements of the load on the power quality, ensure that the power supply frequency and voltage amplitude change, waveform distortion rate and the number of annual power outages are in a small range; on the other hand, the large power grid to the microgrid As a whole, the integration of the grid also puts strict requirements, such as load power factor, current harmonic distortion rate and maximum power are strictly limited.

The energy storage system plays an important role in improving the power quality of the microgrid. Through the inverter control unit, the reactive power and active power provided by the supercapacitor energy storage system to the user and the network can be adjusted, thereby achieving the purpose of improving the power quality. Because supercapacitors can quickly absorb and release high-power electric energy, it is very suitable to be applied to the power quality adjustment device of the micro-grid to solve some transient problems in the system, such as instantaneous power failure and voltage swell caused by system failure. Problems such as voltage dips, etc. At this time, supercapacitors are used to provide fast power buffering, absorbing or supplementing electric energy, and providing active power support for active or reactive power compensation to stabilize and smooth fluctuations in grid voltage.

For uncontrollable micro-power supplies such as wind power generation and photovoltaic power generation, fluctuations in generator output power will degrade power quality. This combination of power supplies and energy storage devices is one of the effective means to address dynamic power quality issues such as voltage dips, inrush currents, and transient power interruptions.

3.4 Optimize the operation of the micro power supply

Green energy sources such as solar energy and wind energy, the nature of their energy sources, determine that these power generation methods tend to have non-uniformities, and the power output is subject to change. As the intensity of wind and sunlight changes, the output of the energy produced by these sources will change accordingly. This requires the use of a buffer to store energy. Since the energy output generated by these energy sources may not meet the peak power requirement of the microgrid, the energy storage device can be used to provide the required peak power in a short time until the power generation is increased and the demand is reduced.

A proper amount of energy storage can make a transition if the DG unit is not functioning properly. For example, during nighttime when solar power is used, wind power is in the absence of wind, or other types of DG units are under maintenance, then the energy storage in the system can play a transitional role, and the amount of energy stored depends mainly on the load demand.

In addition, in the case where the process of energy generation is stable and the demand is constantly changing, it is also necessary to use an energy storage device. A fuel cell is different from wind or solar energy, and as long as it has fuel, it can continuously output stable electric energy. However, load demand varies greatly over time. If there is no energy storage device, the fuel cell will have to be large to meet the peak energy demand, and the cost is too high. By storing excess energy in the energy storage device, it is possible to provide the required peak energy through the energy storage device in a short time. Combined with high energy density materials such as fuel cells, supercapacitors provide fast energy release to meet high power requirements, allowing fuel cells to be used only as an energy source. Combining the power of supercapacitors with fuel cells results in a fuel cell system that is smaller, lighter, and less expensive.

3.5 Improve the economic benefits of the microgrid

The application of energy storage systems is of great significance to the provision of micro-grid economic benefits:

1) Significantly increase the proportion of renewable energy generation, ease the pressure on investing in new transmission and distribution lines, and build new power plants, and reduce system costs;

2) Provide effective spare capacity, improve power quality (faster starting speed than generator), and improve system reliability and stability;

3) Provide an effective load management mechanism to reduce the cost of power supply during peaking, thereby reducing electricity prices and providing economic benefits;

4) In the electricity market, the energy storage system can largely avoid interrupting energy trading and predicting the losses caused by mistakes, thus providing stable electricity prices;

5) Undistributable DG power generation units such as solar energy, wind energy, etc., are greatly affected by natural factors such as weather. DG unit owners cannot formulate certain power generation plans, but with energy storage, they can provide the required time at a specific time. The electrical energy, regardless of how much power the DG unit can emit at this time, only needs to generate electricity according to a pre-defined power generation plan. In the environment of the electricity market, the microgrid and the grid are connected to the grid, with sufficient storage power, the microgrid becomes a schedulable unit, and the microgrid owner can sell electricity to the power company according to different situations, providing peak shaving and emergency power. Support such services to get the most economic benefits.

4. Conclusion

The emergence of supercapacitors solves the contradiction between power density and energy density in energy systems. With the further development of supercapacitor technology, it will gradually replace the batteries that need to be replaced frequently, and the household energy storage system may also be realized. As a kind of energy storage system with huge energy storage, fast charging and discharging speed, wide working temperature range, reliable and safe operation, no maintenance and low price, if it can be widely used in micro-grid, it will promote technological progress and achieve greater Economic benefits.